It is well known to use refractory metal alloys, particularly single-crystal superalloys, to produce components designed to be exposed, in service, to high temperatures. This is thus the case particularly with gas turbine components, such as fixed or moving blades of turbojets.
Components made of a single-crystal superalloy, such as the aforementioned blades, are manufactured by casting, with control of the solidification process.
Inspection of the just-cast components is necessary in order to detect possible defects, such as inclusions, cracks or crazes, which are liable to affect their mechanical behaviour in service.
For this purpose, it is known to carry out macrographic etching of the surface of the components by means of an acid bath. One bath commonly used is a bath containing chloride ions, particularly a bath containing hydrochloric acid and iron perchloride in aqueous solution.
The Applicant has found that the treatment using this bath loses its effectiveness with certain types of superalloys, particularly nickel-based superalloys containing elements such as rhenium or ruthenium. However, the use of these types of superalloys may prove to be necessary in order to allow ever higher service temperatures allowing the efficiency of turbojets to be increased.
It is an object of the invention to provide a method of revealing structure which allows components made of superalloys to be easily and effectively inspected, including when they contain elements such as rhenium or ruthenium.
This object is achieved by means of a method which comprises a macrographic etching step followed, according to the invention, by a xe2x80x9cbleachingxe2x80x9d step by electrolytic etching by means of a bath containing at least phosphoric acid and a weak acid.
The weak acid is, for example, chosen from acetic acid, tartaric acid and citric acid.
Combining phosphoric acid with a weak acid makes it possible to compensate for the selective effect of the etching by the phosphoric acid with respect to certain constituents of the superalloy, a weak acid, such as acetic acid or the like, acting on other constituents, in particular nickel, which is a standard basic constituent of superalloys.
The bath may furthermore contain a constituent capable of making the rate of dissolution during electro-chemical etching uniform, for example a secondary alcohol such as glycol.
According to one particular aspect of the invention, the prior macrographic etching step is advantageously carried out chemically by means of a conventional acid bath containing chloride ions and iron perchloride. Such prior etching of superalloys, especially those containing rhenium and/or ruthenium, causes the component to be blackened because of the persistent formation of rhenium oxide and/or ruthenium oxide on the surface. The electrochemical etching then carried out in accordance with the invention allows the surface to be bleached or cleaned by the removal of these oxides. A visual inspection of the component can then be easily made. Carrying out these twoxe2x80x94chemical and electrochemicalxe2x80x94etching operations, one after the other, makes it possible to combine the effectiveness of each of the baths used with respect to the various elements of the superalloy.
Use of the electrochemical technique allows the reactivity, especially with respect to nickel-based alloys enriched with elements such as Re and Ru, to be increased. The superalloy component is immersed in the bath and arranged so as to be the anode facing a specific cathode having a shape suitable for obtaining a uniform distribution of the lines of current onto the component.